CompileFeatures: Relax cxx_relaxed_constexpr compiler requirements

This in effect means that cxx_relaxed_constexpr is now supported
by MSVC and Intel 18.0-18.04.

Modules/Compiler/Intel-CXX-FeatureTests.cmake

@@ -24,7 +24,7 @@ set(DETECT_CXX14 "((__cplusplus >= 201300L) || ((__cplusplus == 201103L) && !def
 unset(DETECT_BUGGY_ICC15)
 
 set(Intel17_CXX14 "__INTEL_COMPILER >= 1700 && ${DETECT_CXX14}")
-set(_cmake_feature_test_cxx_relaxed_constexpr "__cpp_constexpr >= 201304 || (${Intel17_CXX14} && !(__INTEL_COMPILER == 1800 && __INTEL_COMPILER_UPDATE < 5) && !defined(_MSC_VER))")
+set(_cmake_feature_test_cxx_relaxed_constexpr "__cpp_constexpr >= 201304 || (${Intel17_CXX14} && !defined(_MSC_VER))")
 
 set(Intel16_CXX14 "__INTEL_COMPILER >= 1600 && ${DETECT_CXX14}")
 set(_cmake_feature_test_cxx_aggregate_default_initializers "${Intel16_CXX14}")

Modules/Compiler/MSVC-CXX-FeatureTests.cmake

@@ -9,10 +9,8 @@ set(_cmake_oldestSupported "_MSC_VER >= 1600")
 # https://docs.microsoft.com/en-us/cpp/cpp-conformance-improvements-2017#update_153
 set(_cmake_feature_test_cxx_decltype_incomplete_return_types "_MSC_VER >= 1911")
 
-set(MSVC_2017 "_MSC_VER >= 1910")
-# VS 2017 introduces support for "N3652 Extended constexpr"
-# but as of v15.6 there are still bugs in the implementation
-#set(_cmake_feature_test_cxx_relaxed_constexpr "${MSVC_2017}")
+# VS 2017 v15.3 fixes support for "N3652 Extended constexpr"
+set(_cmake_feature_test_cxx_relaxed_constexpr "_MSC_VER >= 1911")
 
 # VS 2017 Preview introduces support for aggregate initializers.
 set(_cmake_feature_test_cxx_aggregate_default_initializers "_MSC_FULL_VER >= 190024406")

Tests/CompileFeatures/cxx_relaxed_constexpr.cpp

@@ -22,6 +22,7 @@ constexpr int g(const int (&is)[4])
 
 int someFunc()
 {
-  constexpr int k3 = g({ 4, 5, 6, 7 });
+  constexpr int values[4] = { 4, 5, 6, 7 };
+  constexpr int k3 = g(values);
   return k3 - 42;
 }